1. Field of the Invention
This invention relates to a process for preparing a non-yellowing integral-skinned polyurethane foam.
2. Description of the Prior Art
The molded products of integral-skinned polyurethane foam whose core is microcellular and whose external surface is formed integrally with a beautiful skin find their major use in the furniture, automobile assembly, imitation industrial articles, shoes and the like. Conventional integral-skinned polyurethane foam products are yellowed under the influence of light and heat, so that they have been used by coloration to a dark or deep color such as black color in order that the yellowing may not be perceived in appearance. For this reason, kind of color applicable to the products has been limited.
The yellowing of polyurethane foam is ascribable to the use of aromatic polyisocyanates as an ingredient of the polyurethane foam reaction composition. Accordingly, there has been proposed a method for preparing an integral-skinned polyurethane foam by substituting aliphatic or alicyclic polyisocyanates for the aromatic polyisocyanates to prevent it from yellowing (cf. U.S. Pat. No. 4,150,206).
The integral-skinned polyurethane foam thus obtained according to the proposed method, as described with xenon test results, does not yellow and exhibits good weathering resistance in a comparatively low temperature condition of about 45.degree. C., but has drawbacks in further higher temperature conditions that it exhibits a low weathering resistance at above 60.degree. C. and a low heat resistance at above 100.degree. C. The reason why the molded product of the polyurethane foam prepared according to the proposed method is diminished in heat resistance and weathering resistance at higher temperatures is considered to be due to the fact that in the reaction composition for forming polyurethane, a polyether polyol having a functionality of not more than 3 is used as a polyol component and a combination of at least two kinds of amines, organic lead compounds and alkaline metal salts or alkaline earth metal salts is used as a catalyst.